---

In this paper, design, fabrication and test of piezoelectric fiber composite transducer for Lamb wave inspection of fluid loaded plates has been investigated. First, dispersion and wave structure analysis has been performed in order to select proper mode and frequency for fluid loaded plate inspection. The S1 mode with zero out of plane displacement in free boundaries of a plate with predefined thickness has been extracted, afterward a piezoelectric fiber composite comb transducer has been designed and fabricated. Having manufactured the transducer, necessary tests has been done in order to prove that the transducer generated Lamb mode at selected mode and frequency is not affected by the fluid loading, in both pitch-catch and pulse-echo techniques. The tests has been done successfully and then crack detection in fluid loaded plate by means of fabricated transducers has been examined again in both pulse-echo and pitch-catch arrangements. It has been shown that in pitch-catch test, the presence of a predefined crack leads to decrease in S1 mode related peak in receiver. Moreover, in pulse-echo test, due to the interference of crack reflected wave signal with the transducer resonance response, continues wavelet transform, as one of time-frequency signal processing methods, has been employed successfully for crack detection. Consequently, the fabricated signal has necessary efficiency for damage detection in fluid loaded plates using lamb waves.

---

Turbomachinery, specially gas turbines and axial compressors, play an important and vital role in energy producing and transmission industries. Thus, probable defects must be detected and measured in a timely manner, in order to prevent expensive costs of repair and maintenance. In this paper, a gas turbine blade-disc connection model, that is one of the most important and sensitive gas turbine parts, has been inspected using phased array ultrasonic non-destructive testing method. In manufactured mockup specimen, a crack with 4 and 8 millimeters length, has been created in two steps, followed by the length effect study on inspection results. Comparison of acquired signals in different angles for each crack length with acquired signals got from the healthy model in the same angles, has been employed for crack sizing purposes. The increase in the amplitude of crack reflected signals and the decrease in the amplitude of signals related to the wall behind the crack versus increase in the inspection angle, in ultrasonic phased array testing, has been utilized for extracting a novel feature for crack length estimation. Experimental results show that it is possible to evaluate the crack length in the complicated fir-tree geometry of the disc connection area to the blade, with the amount of error less than 10%, using the extracted feature.